1. Field of the Invention
This invention relates in general to a writing control method, a writing control device and an image forming device. More particularly, the invention relates to an image forming device using an intermediate transcriber, such as a copy machine, a printer or a facsimile, etc.
2. Description of the Related Art
An image forming device, such as a copy machine, a printer or a facsimile, etc., is well known in the conventional art. The image forming device includes a scanning and writing device that is used to perform a scanning operation in a main scanning direction for image information according to a main scanning synchronizing signal that is detected after an image forming start signal in the sub-scanning direction is detected, and then to write an image to an image supporter moving in a sub-scanning direction. FIG. 1 shows an example of the above image forming device.
Referring to FIG. 1, the image forming device comprises a optical writing device 100, a drum-shaped photosensor 102, a cleaning device 104 used to clean the photosensor 102, an electrifying device 106 used to electrify the photosensor 102 uniformly, developing devices 5K, 5C, 5M, 5Y, a transfer drum 110, a fixing device 112, transfer paper 114 used as a recording medium, a controller 116 and a paper-feeding device (10) used to feed paper 114. The image forming device further comprises a detecting means (61) used as a means for generating an image forming start signal of the sub-scanning direction. The optical writing device 100 is used as a scanning and writing device, i.e., an exposure device. The drum-shaped photosensor 102, used as an image supporter (a scanned object), is rotationally driven by a photosensor driving means (not shown) to move along the sub-scanning direction to an image writing position, so that an image is scanned by optical writing device 100 in the main scanning direction according to image information and then written onto the photosensor 102. The developing devices 5K, 5C, 5M, 5Y are respectively used to develop an electrostatic latent image on the photosensor 102 into toner images in black, cyan, magenta and yellow. The transfer drum 110 is used as an intermediate transcriber, wherein the transfer drum 110 is rotated by a driving means (not shown) with a rotational speed the same as the photosensor 102 and a mark M is formed thereon. The detecting means (61) is used to detect the mark M on the transfer device 110 so as to generate an image forming start signal of the sub-scanning direction, i.e., the image forming start signal of the sub-scanning direction. The controller 116 receives the image forming start signal of the sub-scanning direction from the detecting means (61) and controls the entire image forming device.
Next, operations related to the aforementioned image forming device is further described. As an image forming operation begins, the surface of the photosensor 102 is electrified to a prescribed potential by the electrifying device 106. The photosensor 102 is rotated in the arrow direction as shown in FIG. 1, and the electrified surface of the photosensor 102 is then repeatedly scanned and exposed in the main scanning direction by a modulated light beams by the optical writing device 100 according to image information of black, cyan, magenta and yellow sequentially. At this time, the photosensor 102 is discharged in a manner that an exposed portion becomes conductive and then the electrified charges flow from an inner face of the photosensor 102 to the ground. An electrostatic line image corresponding to image information each color is sequentially formed on the photosensor 102 by defining that an exposed portion is an image portion and a non-exposed portion is a non-image portion.
Next, the electrostatic latent images corresponding to image information of each color on the photosensor are respectively developed by the developing devices 5K, 5C, 5M and 5Y. Each of the developing devices 5K, 5C, 5M and 5Y has a developer supporter for supporting developer that contains toner of black, cyan magenta and yellow respectively. By applying an immediate potential between a non-image potential and an image potential of the electrostatic latent image on the photosensor 102 from a power device (not shown) to the developer supporter, the selected color toner on the developer supporter is adhered onto the image portion of the photosensor 102. In the example, the developing devices 5K, 5C, 5M and 5Y are installed in a revolving manner. Thus, the four developing devices 5K, 5C, 5M, 5Y are rotated all together by a revolver mechanism (not shown), and in this way, the developing device opposite to the photosensor 102 is circularly altered. By the rotation of the developing devices, one developing device selected develops the electrostatic latent image on the photosensor 102 to form a toner image.
The first color toner image, formed on the photosensor 102 by one selected developing device, is transferred to the transfer drum 110 by a first transfer mechanism (not shown) at a first transfer section, i.e., a close region between the photosensor 102 and the transfer drum 110. As the revolver mechanism (not shown), which is to rotate the developing devices 5K, 5C, 5M, 5Y at one time, finishes the development of the electrostatic latent image corresponding to first color image information on the photosensor 102, the developing devices 5K, 5C, 5M, 5Y are then rotated all together to make one developing device, which is to develop an electrostatic latent image corresponding to second color image information on the photosensor 102, to be opposite to the photosensor 102.
The first color toner image on the transfer drum 110 is further transported to the first transfer section by rotating the transfer drum 110. At this time, each elements of the image forming device in this example is controlled by the controller 116 in such a manner that the second color toner image formed by the developing device on the photosensor 102 reaches the first transfer section, and the second color toner image on the photosensor 102 is transferred at the first transfer section by the first transfer mechanism (not shown) onto the transfer drum 110 so as to overlap with the first color toner image.
When the revolver mechanism (not shown) finishes the development of the electrostatic latent image corresponding to second color image information on the photosensor 102, the developing devices 5K, 5C, 5M, 5Y are then rotated all together to make one developing device, which is to develop an electrostatic latent image corresponding to third color image information on the photosensor 102, to be opposite to the photosensor 102. At this time, each elements of the image forming device in this example is controlled by the controller 116 in such a manner that the third color toner image formed by the developing device on the photosensor 102 reaches the first transfer section, and the third color toner image on the photosensor 102 is transferred at the first transfer section by the first transfer mechanism (not shown) onto the transfer drum 110 so as to overlap with the second color toner image.
When the revolver mechanism (not shown) finishes the development of the electrostatic latent image corresponding to third color image information on the photosensor 102, the developing devices 5K, 5C, 5M, 5Y are then rotated all together to make one developing device, which is to develop an electrostatic latent image corresponding to fourth color image information on the photosensor 102, to be opposite to the photosensor 102. At this time, each elements of the image forming device in this example is controlled by the controller 116 in such a manner that the fourth color toner image formed by the developing device on the photosensor 102 reaches the first transfer section, and the fourth color toner image on the photosensor 102 is transferred at the first transfer section by the first transfer mechanism (not shown) onto the transfer drum 110 so as to overlap with the third color toner image.
On the other hand, a transfer paper 114 is fed to resist rollers from a paper feeding device (10), and the resist rollers send out the transfer paper 114 accompanying with the full color image on the transfer drum 110. As a full color image is formed on the transfer drum 110, a receded or stopped secondary transfer mechanism (not shown) is activated, and then the full color image on the transfer drum 110 is entirely transferred to the transfer paper 114 (from the resist roller) by the secondary transfer mechanism. The full color image that has been transferred on the transfer paper 114 is fixed by the fixing device 112, and then the transfer paper 114 is ejected out of the image forming device.
FIG. 2 shows a structure of the optical writing device 100 in FIG. 1. The optical writing device 100 comprises a light source 120. The light source 100 is sequentially modulated by a modulating means (not shown) according to image information of prime colors, such as black, cyan, magenta and yellow. Then, a laser beam, which is sequentially modulated by image information of black, cyan, magenta and yellow, is emitted.
The laser beam from the light source 120 is collimated by a collimator lens (15), and then deflected by a deflection reflection surface of a rotational polygon mirror 122 (as a scanning means). The rotational polygon mirror 122 is rotationally driven by a driving means (not shown) to scan repeatedly in the main scanning direction. The laser beam from the rotational polygon mirror 122 is converged by an imaging lens 124 and then is imaged on the photosensor 102 as a laser spot. By using that the rotational polygon mirror 122 is rotationally driven by the driving means (not shown), the laser spot scans the photosensor 102 repeatedly in the main scanning direction to form an electrostatic latent image on the photosensor 102.
An light receiving element 126 as a main scanning synchronizing signal generating means is arranged out of an image range that is within a laser beam scanning range. The light receiving element 126 receives a laser beam from a polygon mirror 122 and then detects it, so as to generate a main scanning synchronizing signal that determines a recording start position (lateral resist) in the main scanning direction.
On the other hand, an image forming start signal of the sub-scanning direction (i.e., an image forming start signal of the sub-scanning direction), which determines a recording start position (vertical resist) in the sub-scanning direction (i.e., an image forming start position in the sub-scanning direction), is detected and generated by such as a light receiving means to detect a reflection light or a transmission light that is obtained by irradiate a light beam to the mark formed on the transfer drum 110 and the mark formed on the photosensor 102, a rotation start timing of the resist roller, a detection signal from a paper detecting sensor that is used to detect the transfer paper 114 right after the resist roller, a rotary encoder built in a photosensor driving means, etc. There are many methods to generate the image forming start signal of the sub-scanning direction, but in this example, the image forming start signal of the sub-scanning direction is generated by that the detecting means (61) detects the mark M formed on the transfer drum 110.
The main scanning synchronizing signal from the light receiving element 126 and the recoding start signal of the sub-scanning direction that comes from the detecting means (61) are transmitted to the controller 116. Then, the controller 116 instructs the optical writing device 100 to perform an optical writing (exposure) operation onto the photosensor 102 according to the main scanning synchronizing signal from the light receiving element 126 and the recoding start signal of the sub-scanning direction from the detecting means (61).
FIG. 3 shows a timing diagram of an operation in the above exemplary description. For convenience, t represents time, a time when the image forming start signal of the sub-scanning direction from the detecting means (61) is detected by the controlled 116 together with the optical writing operations corresponding to image information of colors is defined as t=0, and a time interval for the light receiving element 126 to generate the main scanning signal is represented by T. When an optical writing operation corresponding to first color image information is performed by the optical writing device 100, a time when the controlled 116 detects initially the main scanning synchronizing signal from the light receiving element 126 after t=0 is represented by t1. When an optical writing operation corresponding to second color image information is performed by the optical writing device 100, a time when the controlled 116 detects initially the main scanning synchronizing signal from the light receiving element 126 after t=0 is represented by t2. When an optical writing operation corresponding to third color image information is performed by the optical writing device 100, a time when the controlled 116 detects initially the main scanning synchronizing signal from the light receiving element 126 after t=0 is represented by t3. When an optical writing operation corresponding to fourth color image information is performed by the optical writing device 100, a time when the controlled 116 detects initially the main scanning synchronizing signal from the light receiving element 126 after t=0 is represented by t4. In this description, the transmission time for each signal is ignored.
As an initial main scanning synchronizing signal from the light receiving element 126 is detected after the image forming start signal of the sub-scanning direction from the detecting means (61) is detected, the controller 116 instructs an optical writing (exposure) operation to the optical writing device 100. FIG. 5 shows an operation flow chart in the above example. At Step 1, the controller 116 checks regularly the image forming start signal of the sub-scanning direction coming from the detecting means (61) to determines as to whether the image forming start signal of the sub-scanning direction is detected. At Step 2, if the image forming start signal of the sub-scanning direction is detected, this time t is set as 0. Next, at Step 3, the controller 116 checks regularly the main scanning synchronizing signal coming from the light receiving element 126 to determines as to whether the main scanning synchronizing signal is detected. At Step 4, if the main scanning synchronizing signal is detected, the controller 116 instructs an optical writing (exposure) operation to the optical writing device 100. The above operation flow is independently performed with an optical writing operation corresponding to image information of each color.
In the image forming device described above, because the main scanning synchronizing signal and the image forming start signal of the sub-scanning direction are not synchronized in general, when the image forming start signal of the sub-scanning direction from the detecting means (61) reaches the controller 116, angles of the rotational polygon mirror 122, which are respectively for when the optical writing corresponding to image information of the first color is started and for when the optical writing corresponding to image information of the second color is started, are different. Namely, when the image forming start signal of the sub-scanning direction from the detecting means (61) reaches the controller 116, the angles of the rotational polygon mirror 122 when the optical writing corresponding to image information of each color is started are not equal to each other.
Therefore, as shown in FIG. 3, t1, t2, t3 and t4 are not same, and ranges between t=0 and t=T. As a result, the toner image of each color in the sub-scanning direction occurs in a color deviation. For example, as shown in FIG. 3, when the time difference between t1 and t2 is large, such as the optical writing corresponding to image information of the first color and the optical writing corresponding to image information of the second color, the start time of the optical writing is shifted close to T. As a result, the toner image of the first color and the toner image of the second color are shifted close to one line as shown in the lower part of FIG. 3.
In addition, in the specification, “line” means a pixel set that the positions in the sub-scanning direction are equal among the pixels forming image information. During the image formation, from the first scanned line to the subsequently scanned lines, these lines are represented by the first line, the second line, the third line, etc. Even though a scanning and writing device to form a plurality of lines by scanning once, each of lines is represented by the first line, the second line, the third line, etc. as shown in FIG. 21.
As a technology to avoid the aforementioned color deviation, there is a method to control the exposure by determining as to whether t1 to t4 are equal to or larger than a prescribed value. In this method, for example, when t1 is equal to or larger than T/2, the optical writing (the exposure) is started at time t1. When t1 is less than T/2, the optical writing (the exposure) is started at time t1+T. When the exposure is started at time t1+T, the optical writing device can be stopped at time t1, or the optical writing device can be still activated without emitting a laser beam.
When this conventional method is applied to a situation shown in FIG. 3, the position relationship of each color is as shown in FIG. 4. When the exposure is started at time t1+T, the dot represented by dash line shown in FIG. 4 is a dot at the time t1 that the exposure is not performed. However, according to this method, when time t1 is right before time T/2 and time t2 is right after time T/2, the toner image of the first color and the toner image of the second color cannot be avoided from being shifted close to one line.
In addition, there is a disclosed image forming device in Japanese Laid Open No. 11-212009, in which the above method and a multi-beam technology are combined together. However, this image forming device is to reduce a position shift of image information of the first line, rather than to avoid toner image of each color from being shifted close to one line.
In the conventional image forming device, when the electrostatic latent image is formed by the writing device, a dot position shift occurs easily in the sub-scanning direction. As the dot position shift occurs, a color deviation occurs when overlapping each of the color images on the intermedium transfer body. Therefore, the image quality is degraded and the original image cannot be truly reproduced.